This invention relates to a diaphragm control system for interchangeable lenses with which there is possible, optionally, either a manual adjustment of the diaphragm or an adjustment which is effected automatically by means of a camera with an automatic exposure mechanism.
It is already known to use so-called instant-return diaphragms which are released by the camera mechanism just before the exposure and then close to a value which has been manually selected by the user. The control finger which engages into the camera is in this case connected directly with a slotted ring, and upon the actuation of the diaphragm, it carries out a movement referred to below as non-linear, i.e. for equal diaphragm intervals the control finger moves over unequal paths or angular steps which become constantly smaller the further the diaphragm closes. In the case of lenses with instant-return diaphragm no special demands are made as to the course of movement of the control finger or diaphragm blades, it being merely essential that the slotted ring move from the end abutment provided for maximum opening to the end abutment corresponding to the aperture which has been selected.
This end abutment which is adjusted by means of the diaphragm preset ring on the lens is generally driven via a movable cam which converts the linear movement of the diaphragm preset ring to an adjustment movement which corresponds to the non-linear movement of the slotted ring.
If such a lens is to be used in combination with a camera which automatically adjusts the diaphragm to a value calculated on basis of the predetermined exposure time and the measured conditions of light, then the non-linear movement characteristic of the control finger makes itself felt in an especially detrimental manner. In particular, with a small diaphragm aperture the extent of movement of the control finger between two adjacent diaphragm intervals or settings is so small that an accurately reproducible association between the diaphragm value selected by the camera mechanism and that actually transmitted by the control finger to the diaphragm is no longer possible with the desired accuracy, due to manufacturing tolerances and looseness in the coupling between camera and lens assembly containing the diaphragm.
A proportional enlargement of the stroke of the control finger in order to circumvent these difficulties is undesirable since the speed of closing of the diaphragm is reduced if longer paths of movement are provided. Furthermore, the non-linear movement characteristics, which are different for different types of lenses, give rise to difficulties upon the conversion of the calculated electric value into the position of the control finger corresponding to the true diaphragm value.
Known interchangeable lens systems with automatically adjustable diaphragm operate with a so-called "diaphragm stop" which is arranged in the housing and when the automatic diaphragm mechanism is set serves as an abutment for the control finger instead of the abutment which is displaceable in the lens housing upon manual presetting of the diaphragm. The position of the diaphragm stop is, corresponding to the movement characteristic of the control finger, dependent also in non-linear manner on the diaphragm value calculated in the camera and transmitted via electric-motor converter and mechanical transmission to the diaphragm stop. Here therefore the linearization of the control movement takes place in the camera, preferably electronically. Although this represents an advantageous solution as viewed from the standpoint of the space in the lens and the camera housing, the problem arises here of the difficulty of reproducible adjustment of small diaphragms since the coupling between a non-linearly moved control finger and the diaphragm stop takes place at the camera/lens interface.